Slingshot effect

From Conservapedia

Slingshot effect is acelerating of an object that travels through the universe by other objects of the universe transfering their momentum to the traveling object through gravitational interaction with it.

E.g. when cosmic probe is passing a planet in direction of movement of this planet the gravitational interaction between the probe and the planet accelerates the probe (pushes the probe forward) and the probe gains kinetic energy during the encounter with the planet.

Slingshot effect is used in practice to put cosmic probes on higher orbits up to pushing them out from the Solar system.